A computer system power supply generates heat when providing power to the computer system. To improve the power supply's performance, and to protect components in the computer system, it is known that a power supply fan can be used to blow air over and cool the power supply.
Typically, a computer system power supply operates in either a main power-on state, i.e. a "full power" mode, or main power-off state, i.e. a "standby power" mode. In the full power mode, the main power supply outputs are enabled to provide power to the components in the computer system. In the standby power mode, fewer computer system components are operational and, therefore, less power needs to be provided by the power supply.
FIG. 1 illustrates a known power supply fan circuit for a computer system. A power supply (PS) fan 10 is coupled between the power supply's main 12 volt (V) line 20 and ground. The load placed on the power supply in full power mode by the components in the computer system is represented by a resistor R125 coupled between the main 12 V line 20 and ground. The load placed on the power supply in standby mode, when less than all of the components are operational, is represented by a resister R235 coupled between a 5 V standby (SdBy) line 30 and ground. The 5 V standby line 30 may be provided, for example, by a standby auxiliary converter in the power supply. Note that the power supply fan 10 is not coupled to the 5 V standby line 30.
When the power supply is in the fall power mode, the power supply fan 10 receives power from the main 12 V line 20. This causes the power supply fan 10 to blow air over and cool the power supply. When the power supply is in the standby power mode, no power is provided to the power supply fan 10, which therefore does not blow air over or cool the power supply. This typically has been acceptable because a power supply may generate less heat in the standby power mode, and therefore may not need to be cooled.
Some computer systems, however, require that a power supply provide more power in the standby power mode as compared to other computer systems. In such computer systems, additional heat sinks or fans have been used to cool the power supply when in standby power mode. These solutions, however, increase the cost and size of the power supply unit.
Another problem associated with fans in a computer system relates to system fans that are used to cool components in the computer system. For example, FIG. 2 illustrates a known system fan circuit for a computer system. Two system fans 40, 50 are each coupled between a 12 V power supply output 60 and ground. The capacitance of the power supply is represented by a capacitor C065 that is coupled between the 12 V output 60 and ground.
The operation of each system fan 40, 50 draws current from the power supply in pulses at a frequency of, for example, 100 hertz (Hz). This type of current draw creates a step-loading condition to the power supply, causing a power supply "ripple voltage" to be distributed to other components in the computer system. That is, a pulse of current will cause a momentary voltage drop because the power supply has an equivalent internal resistance. The higher the magnitude of this current pulse, the greater the voltage drop at the power supply terminals. A feed back loop eventually regulates out this voltage drop, but the resulting fluctuations in the voltage signal is referred to as a "ripple." Because the frequency of the current pulse from the two system fans may not be equal, periodically both system fans may draw current simultaneously. This will further increase the ripple noise in the system.
The operation of some computer system components can be disrupted when the ripple noise becomes too large. For example, a hard disk drive may require that the system's ripple peak-to-peak magnitude, including transient response, not exceed 0.8%. To reduce ripple noise, a traditional filter, such as a passive Resistance-Capacitance (RC) or Inductance-Capacitance (LC) filter, may be used. For example, to bring ripple noise down to a required level, a 16 V-12,000 microfarad (.mu.F) capacitance may need to be placed on the 12 V bus. This, however, significantly increases the cost of the computer system.
In view of the foregoing, it can be appreciated that a need exists for a computer system fan circuit that solves the problems discussed above.